I'm a vet. Toxocara eggs have an average diameter of about 70 microns, apparently, which makes them about 10 times bigger than some other much commoner parasites found in water, such as Cryptosporidia. Things like Crytosporidia are removed from drinking water by filtering. Any filter which does a fair job of removing the smaller parasites is going to find it very easy to remove Toxocara, so if someone's worried about Toxocara in drinking water, logically you should worry more about other parasites that are smaller.
Also, any water that comes from eg a reservoir may well have been contaminated by foxes, which also carry Toxocara and which certainly haven't been wormed. The same foxes will also be leaving faeces all over the parks, as will the unwormed dogs of the irresponsible owners, of course. Direct contamination of the park is a far more likely source of infection for any child, therefore.
Compared with all this, I think the chances of a Toxocara egg getting into a child via the drinking water, rather than any other way, are so small as not to be worth worrying about, but if the OP is worried, all she needs to do is either boil the drinking water or use a home filtration system that removes things over 20 microns or so diameter, and there can be no possible risk.
While we are on worm education: the Toxocara eggs are not infective until some time after they have been passed by the dog - about a couple of weeks. Therefore, although there are many reasons why hygiene around dogs and their faeces is wise, contact with fresh faeces or a dog itself will not put a child at any risk of contracting Toxocara.
Also, Toxoplasma (the pregnant women and cat litter trays one) is something completely, totally different.